Crashes of Asteroids Shed Tiny Diamonds In Glittering Trillions

By WILLIAM J. BROAD

Published: June 11, 1996

SPEEDING rocks from space that have slammed into Earth periodically over the ages are usually seen as agents of destruction and death, begetting global palls of dust that block sunlight, disrupt the climate and do in creatures like the dinosaurs.

But it turns out that the same upheavals that blast craters in solid granite and spew tons of incandescent rock into the sky also tend to produce diamonds, trillions of them, more than any thief ever dreamed of.

Alas for gem lovers and investment bankers, most of the impact diamonds are microscopic. But some are larger -- the size of match heads or peanuts -- and are considered good enough for industrial use, if not jewelry.

But over all, experts say, the discovery is likely to be more of a boon for understanding Earth's cataclysmic past than for commercial diamond mining. That history is important because periodic strikes from space are thought to have repeatedly shaped life's evolution, killing off some species and making room for others. Yet such violent episodes tend to be surrounded by mystery and debate.

Scientists have now found diamonds in more than a half-dozen impact zones around the globe and are hunting for other places where the enormous heats and pressures of cosmic collisions have turned plain old carbon into the hardest substance known to humans. To date, it appears that no crater examined for diamonds has failed to reveal them.

"The question is whether diamond is going to be a ubiquitous phase in the ejecta," Dr. Peter S. Fiske, a planetary scientist at the Lawrence Livermore National Laboratory in California, said in an interview. "It's just a question of going to look at these other craters."

The American Geophysical Union, the world's largest professional group devoted to earth studies, based in Washington, recently held its first meeting on impact diamonds and mineralogy, which Dr. Fiske helped organize.

The rocks of old craters, experts say, probably hide thousands and perhaps millions of tons of diamonds. The rub is getting them. Candidate rocks can be very deep, buried over the eons by such geological processes as erosion and sedimentation.

And even when in hand, raw rocks have to sit for weeks and months in caustic baths that slowly eat away softer rock with deadly cocktails of nitric, hydrochloric, perchloric and sulfuric acids, among other nasty substances. But the residue is pure diamond.

Western experts say that the Russians have investigated mining impact craters for diamonds in remote parts of Siberia and that Western companies are quietly conducting similar studies.

"It may be that a lot of craters have diamonds," Dr. Richard A. F. Grieve, a geophysicist at the Geological Survey of Canada who is a leading crater-identification expert, said in an interview. "You never know, in the right place they might be commercial grade."

One company, Dr. Grieve added, is now looking for gemstones in a North American crater. "They don't think it's commercial, but they don't want to get caught with their pants down," he said, adding that he was not allowed to disclose the company's name.

The world's largest known crater, named Sudbury after a nearby town, is a giant scar some 155 miles across in Ontario, Canada.

Earth's face is marred by 150 or so authenticated impact craters. More are added at the rate of up to five a year as geologists find new evidence of collision fury. But some candidates repeatedly fail to make the list. The problem is knowing how to distinguish the scars of cosmic impacts from those made by volcanoes and earthquakes and gentler processes like sedimentation and glacial action.

Diamonds are seen as a likely new way to identify eroded impact craters and to investigate the planet's violent past. The durability of diamonds, experts say, may shed light on extremely old impact scars that have lost their other characteristic features.

"There's no reason they can't to go back to the Cambrian," Robert M. Hough, a diamond-crater expert at the Open University in England, said in an interview of impact diamonds from the geological age 600 million years ago when marine creatures ruled the earth.

"Diamond is very interesting," agreed Dr. Virgil L. Sharpton, a crater expert at the Lunar and Planetary Institute in Houston. "It helps distinguish features produced by these very high-pressure impacts from all the other things they can be confused with. That's a tremendous tool."

Scientists estimate Earth's surface has been struck by perhaps hundreds of thousand asteroids and comets big enough to carve out craters more than a half-mile wide. Most of those craters have vanished without a trace in the past few billion years, often carried deep into Earth's interior through geologic action.

Dr. Sharpton said new signposts like diamonds might eventually quadruple the number of known craters, raising the total from 150 to 600 or more. "There are plenty out there to be discovered," he said.

Diamonds are traditionally thought of as travelers from Earth's deep interior, from depths of roughly 100 miles and perhaps as deep as 300 miles. How they form there is well known. Crushing pressures and blistering heats work in unison to squeeze carbon into stones of unrivaled hardness.